Study On The Characteristics And Mechanism Of Middle Fracture Expansion Under Simulating The Acidizing Fracturing Process Of Hole Material

The carbonate reservoirs are widely distributed in China,with large block area and abundant oil and gas reserves in Shunbei Oilfield in Aksu area,Xinjiang.However,the geological conditions of the block are complex,and the hole and fracture structure are developed.Because of the limitation of core sample size of carbonate reservoir,acidizing fracturing can not be carried out,and the characteristics of fracture hole in carbonate outcrop sample are irregular,so it is difficult to quantify the expansion and development of fracture in fracture structure.Therefore,it is necessary to prepare a similar material physical model which is similar to carbonate rock and can simulate fracture structure in internal spatial distribution.In this paper,the mineral composition,microstructure and mechanical strength of different carbonate rocks are analyzed,and the microstructure and mechanical characteristics of carbonate rocks before and after acid-rock reaction are revealed.Through acid fracturing test,the effect of acid-liquid displacement rate,stress difference and fracture distribution position on fracture propagation and fracture morphology is revealed,and the acid-induced fracture description model is established;Finally,the macroscopic fracture propagation and fracture characteristics are analyzed from the damage mechanism of microscopic acid rock reaction.The results of this paper are as follows.1)XRD results show that the higher the carbonate content,the faster the acid rock reaction rate,and the faster the filling reaction rate in the carbonate fissure area than the matrix reaction rate.The surface acid erosion of fractured limestone is inhomogeneous acid erosion mode,and the local strength and local strength damage rate of limestone is larger than that of fracture area.2)According to the difference of acid erosion between mineral composition and fracture area,a physical model of simulated fracture body was established by selecting the mix ratio of similar materials and designing the structure of fracture-cavity.The carbonate content of this model is more than 90%.The acidified fracturing test can be satisfied by arranging different fracture structures inside the sample.3)By comparing the pump pressure curve and fracture shape characteristics of the sample,the analyzed the fracturing form and fracture expansion form.The results show that the fracture can be divided into dissolution type and fracturing dissolution type because of the difference of fracture position.4)The analysis of the microscopic characteristics of acid erosion in the fracturing test samples shows that,the acid erosion results in the expansion of the plastic zone at the crack tip and the decrease of the rupture pressure,and the maximum rupture pressure decreases gradually with the increase of fracturing time.The time effect of acid pressure damage is proved.5)The analysis of the characteristics of fracture expansion shows that,with the increase of horizontal lateral pressure coefficient,the fracture complexity decreases and the crack expands single;At low displacement rate,the acid-pressure fracture tends to expand downward along the fracture zone in the direction of gravity.Compared with the variation law of pump pressure curve crack and joint morphology at different displacement rates,the faster the sample displacement rate,the greater the rupture pressure,and the less obvious the acid erosion characteristics such as vermis hole and dissolution hole on the crack surface.